The present invention generally relates to tread patterns for pneumatic tires, particularly to a variation of block tread patterns designed to balance the rate of wear between the leading edge or heel and the trailing edge or toe of the tread blocks.
A pneumatic tire includes a ground contacting portion or tread, the tread having a pattern designed to provide the tire with a desirable combination of traction, durability, ride comfort and quiet operation. It is also desirable that the tread pattern provide the tire with an all-weather capability, that is a set of characteristics providing adequate performance under a variety of adverse road conditions including snowing, freezing, raining, etc.
Tire tread patterns designed for traction on wet surfaces, snow and ice often feature a block type tread pattern. A block type tread pattern is characterized by a plurality of main grooves extending in a circumferential direction and a number of lateral grooves extending in a more or less axial direction. The areas of tread between the circumferential and lateral grooves are referred to a tread blocks. Tread blocks may also be defined by the edges of the tread and by grooves having other orientations. In comparison, rib-type tread patterns are characterized primarily by circumferential grooves separating circumferentially continuous ribs. Tread designs may also combine rib and block patterns.
The use of blocks as elements of a tread pattern tends to increase the level of noise generated by such tires as compared to rib-type tires. Also, as noted by U.S. Pat. No. 5,538,060, such blocks have a tendency towards irregular wear due primarily to their lack of stiffness in the circumferential direction of the tread.
It is known in pneumatic tires having a block tread pattern that normal operation of the tire produces uneven wear of the tread blocks called heel-and-toe wear. In heel-and-toe wear, the rate of wear at the toe or trailing edge of the blocks exceeds the rate of wear at the heel or leading edge of the blocks. In normal operation, the heel of each block strikes the pavement first followed by the toe. Similarly the heel of each block is lifted first from its contact with the pavement followed by the toe. In addition to reduced tread life, heel-and-toe wear increases the level of noise generated by the operation of the fire. Also, the cornering and braking performance of a tire with heel-and-toe wear may be degraded.
U.S. Pat. No. 5,891,276 discloses a variation of the block tread pattern designed to suppress heel-and-toe wear wherein a narrow block is provided outside each block, the narrow block having a surface formed to be a circular arc by setting both end parts of the narrow block to be lower than he adjacent tread block by 1.5 to 2.5 mm.
Several design patents, for example U.S. Des. 335,111 and U.S. Des 309,442 provide example of block tread patterns featuring a notch or groove in the toe area of some tread blocks. As taught by U.S. Pat. No. 4,122,879 (""879), heel-and-toe wear is severely affected by the configuration of the lateral grooves, particularly the configuration of the grooves defining the blocks in the vicinity of the edges of the tread and more particularly to the angle of the walls of the lateral grooves relative to the equatorial line. Patent ""879 discloses a design to mitigate heel-and-toe wear by providing the walls of lateral grooves with an angle of from 20 to 40 degrees relative to the central axis of the lateral grooves.
Despite recognition of the problem of heel-and-toe wear and various solutions described by the prior art, there remains an opportunity for tread pattern designs to mitigate heel-and-toe wear problems in block-type tread patterns and to provide fires with a balance of traction, durability and noise generation characteristics suitable to variety of vehicles and road conditions.
The present invention to provides an improvement that is generally applicable to the design of block tread patterns for pneumatic tires and particularly applicable to directional block tread patterns having the capability of balancing heel-and-toe wear.
To balance the rate of heel and toe wear, the leading edge or heel of one or more blocks are provided with one or more notches, the notches having a variable width in the axial direction, the width generally decreasing from a maximum at the heel to a minimum in the direction of the toe. Said notches provide the tread blocks with a variable net to gross where the net to gross increases from the heel to the toe of the blocks.
The variation of net to gross from the heel to the toe of a block can be provided by a wide range in the number shape and dimensions of the notches that can be applied to the toes of a blocks. For example the notches may be rectangular, triangular, trapezoidal or curved in shape. The notches may also have variable depth, having a maximum depth at the heel of the block.
According to the invention, a pneumatic tire has a directional tread with blocks. Each block has a leading edge or heel and a trailing edge or toe extending from a ground contacting tread surface. One or more of the blocks is characterized by one or more notches originating at the heel and extending into the block from the tread surface. The notches are rectangular, triangular, trapezoidal or curved in shape. The notches extend in the circumferential direction toward the toe whereby the area of the tread surface has a variable net to gross that increases from the leading edge to the trailing edge of the one or more blocks. The notches have a length ranging from 40% to 70% of the length of the block and a variable width in the circumferential direction with the greatest width being at the heel and ranging from 10% to 30% of the width of the block. The notches also have a variable depth in the radial direction with the depth being greatest at the heel.
The toe of the blocks are sometimes provided with one or more sipes. The sipes have a circumferential orientation and a length equal ranging from 5% to 35% of the length of the block.